Hitherto, high-hardness sintered bodies using cubic boron nitride have been known. A cBN particle is higher in hardness, strength and toughness than ceramic binders, serves as a skeleton in the sintered body, and serves to hold such material strength as to enable enduring the cutting of high-hardness hardened steel. In contrast, ceramic binders serve to enable a cBN particle, which is a difficult-to-sinter material, to be sintered at industrial-level pressure and temperature, and simultaneously impart the effect of suppressing chemical/thermal wear because the ceramic binders have lower reactivity with iron than that of cBN.
The cBN sintered bodies are roughly divided into two categories: one that has a high cBN content and in which cBN particles are bonded to each other and the remainder is made of a binder containing Co and/or Al as a main component (hereinafter abbreviated as high cBN content sintered body); and the other that has a relatively low cBN content and in which cBN particles have a low ratio of contact with each other and are bonded together interposing ceramics having a low affinity with steel and made of Ti nitride (TiN) or Ti carbide (TiC) therebetween (hereinafter abbreviated as low cBN content sintered body).
The former, the high cBN content sintered body achieves outstanding stability and long life due to cBN's excellent mechanical characteristics (high hardness, high strength, high toughness) and high thermal conductivity in applications where chips are broken down and unlikely to generate shear heat, and is suitable for the high-efficiency cutting of an iron-based sintered part governed by mechanical wear and damage due to abrasion against a hard particle as well as of gray cast iron governed by damage due to thermal shock in high-speed continuous cutting. However, the high cBN content sintered body has shorter life in processing hardened steel and the like where a continuous chip generates a mass of shear heat that exposes a cutting edge to high temperatures, because cBN components thermally worn by iron cause more rapid development of wear than that in conventional cemented carbide tools and ceramic tools.
In contrast, the latter, the low cBN content sintered body demonstrates excellent wear resistance because of the effect of the binder that has a lower affinity with steel under high temperatures than that of cBN and made of TiN or TiC ceramics, and has opened up the market of hardened steel cutting as a cutting tool that can achieve ten times to several tens of times longer tool life than that of conventional tools particularly in processing hardened steel that could not be practically processed with conventional cemented carbide tools and ceramic tools.
A variety of these cBN sintered bodies have been developed.
For instance, Japanese Patent Laying-Open No. 2000-044347 (PTL 1) and Japanese Patent Laying-Open No. 2000-044350 (PTL 2) describe a cBN sintered body consisting of 45 to 70 volume % cBN particles and a binder phase containing at least one selected from the group consisting of: carbides, nitrides, carbonitrides, and borides of group 4a, 5a and 6a elements; nitrides, borides, oxides, carbonitrides, and borides of Al; and a mutual solid solution thereof, where the mean value and the standard deviation of the thickness of the binder phase are defined.
In addition, Japanese Patent Laying-Open No. 2002-302732 (PTL 3) describes a cBN sintered body consisting of an ultrafine-grained cBN particle having a particle size of not less than 0.01 μm and not more than 0.70 μm and a binder phase consisting of at least one of: group 4a, 5a and 6a elements; Al; carbides, nitrides, and borides thereof; and a mutual solid solution and a mixture thereof, wherein the mean value of the thickness of the binder phase is defined.
As mentioned above, in the high-efficiency cutting of cast iron, the high cBN content sintered body has been in practical use. In recent years, the enhancement in the performance of automobile engines is accompanied by the progress of the adoption of aluminum in cylinder blocks aimed at reducing weight. For a liner that is a portion of a cylinder block where a piston slides, cast iron that has excellent strength and wear resistance is employed, and centrifugally cast iron that excels sand mold cast iron in mass productivity has been increasingly employed in recent years. Centrifugal casting is a technique of rotating a casting mold in casting a liner such that centrifugal force is utilized to make a hollow casting without the use of a core.
The centrifugal casting above, which, in principle, rapidly cools a contact portion and an innermost diameter portion of the casting mold and thus causes a dendrite structure or a rosette structure having very poor machinability to be created in these areas, has a problem that tool life is significantly deteriorated when the cast iron structure of a portion to be cut is the above-described dendrite structure or rosette structure in finishing an internal diameter as desired through cutting. It also has a problem of unstable tool life because a cast iron structure greatly varies depending on the production lot of a liner. As such, the recent fact is that conventional high cBN content sintered bodies have insufficient wear resistance and cannot provide satisfactory tool life.
In view of such a situation, WO 2008/087940 (PTL 4) employs Al2O3 as a main component of a binder phase and adds appropriate quantities of ZrO2 and TiC to stabilize tool life in the centrifugally cast iron processing; however, satisfactory tool life has not been obtained.